Glioblastoma's immune shield runs on fructose - and removing the sugar transporter stopped tumors cold

Glioblastoma has a five-year survival rate below 7%. The standard of care has barely changed in two decades. Immunotherapy, which has transformed treatment for many cancers, has largely failed against this brain tumor because glioblastoma is extraordinarily good at shutting down the immune system in its immediate vicinity.

Now Northwestern Medicine scientists have found how it does it - and the answer involves a sugar that, in the rest of the body, is associated with inflammation, but in the brain does precisely the opposite.

Microglia, fructose, and a transporter called GLUT5

The study, published March 17 in the Proceedings of the National Academy of Sciences, focused on microglia - immune cells that are resident in the brain and normally protect the central nervous system. In glioblastoma, microglia are part of the tumor microenvironment, the cellular neighborhood surrounding the cancer. The researchers found that these microglia uniquely express a fructose transporter called GLUT5, and that they are the only immune cells in the glioblastoma microenvironment capable of metabolizing fructose.

That metabolic capability turns out to be critical. When microglia process fructose, it shifts them into an immunosuppressive state - they stop attacking the tumor and instead help it grow. The fructose metabolism effectively creates an immune shield around the glioblastoma.

"We knew microglia use this fructose transporter as part of their normal biology, but we did not expect it to be this important for brain tumor growth," said Jason Miska, assistant professor of neurological surgery at Northwestern's Feinberg School of Medicine and the study's senior author.

Removing the transporter unleashed the immune system

The dramatic result came when the researchers genetically deleted GLUT5 in mouse models. Without the fructose transporter, the microglia could no longer metabolize fructose - and the immune suppression collapsed. The tumors showed dramatically stronger immune responses: better recognition of tumor cells, increased production of inflammatory cytokines, and rapid multiplication of CD8+ T cells, the immune system's primary cancer-killing cells.

"Across several mouse models, when we removed the fructose transporter, the tumors simply didn't grow," Miska said. "It was far more dramatic than we anticipated."

The effect cascaded through the immune system. It was not just the microglia that changed behavior. T cells and B cells within the tumor also became more activated, producing more inflammatory molecules. Co-first author Leah Billingham described it as "an intricate interaction between the different parts of the immune system" - blocking fructose metabolism in one cell type triggered a broader immune awakening.

Fructose acts differently in the brain

One of the study's most surprising aspects is how fructose behaves in the brain compared to the rest of the body. In peripheral tissues, high fructose consumption is linked to inflammation and is associated with conditions including colon cancer and diabetic neuropathy. In the brain, the researchers found, fructose does the opposite - it suppresses inflammation.

"Fructose consumption is associated with so many bad inflammatory outcomes in patients," Miska said. "What's interesting here is that in the brain, it seems to be working differently. It still helps the brain tumor grow, but now we're seeing something very different in the brain than we see in the rest of the body."

This tissue-specific effect is important for therapeutic development. A drug that blocks fructose transport in the brain would need to be carefully targeted to avoid unintended effects elsewhere.

From mouse models to drug targets

The findings are in mice, and the gap between a genetic knockout in a mouse model and a drug that works safely in a human brain is substantial. The team's next step is to identify small-molecule inhibitors of fructose transport that could be tested in preclinical settings, combined with existing standard-of-care treatments or immunotherapies.

"Once we can get our hands on something that is promising as a fructose transport inhibitor, we will then take it into preclinical stages where we add standard-of-care therapies for brain tumors or immunotherapies and see if we can sensitize them," Miska said.

Whether blocking fructose metabolism can crack glioblastoma's immunotherapy resistance in patients remains an open question. But the identification of a previously unknown immune suppression mechanism - one with a clear molecular target in GLUT5 - provides a new angle of attack against a tumor that has resisted almost everything else.